Reliable Multicast Protocol with Packet Forwarding in Wireless Internet
Taku NOGUCHI
†, Toru YOSHIKAWA
‡and Miki YAMAMOTO
‡†
College of Information Science and Engineering, Ritsumeikan University
1-1-1, Nojihigashi, Kusatsu-Shi, Shiga, 525-8577, Japan, noguchi@is.ritsumei.ac.jp
‡
Department of Communications Engineering, Graduate School of Engineering, Osaka University
ABSTRACT
One of the most important problems in reliable multicast communication is reducing generated redundant NAKs to avoid NAK implosion. A number of NAK suppression mechanisms which try to resolve this technical problem have been pro-posed thus far. In wireless multicast which includes mobile hosts with wireless access channel, packet losses occur be-cause of not only low quality of wireless communication chan-nel but also tentative shutdown of connection due to handover. When remote subscription which enables effective usage of network resources is applied, a host re-joins to a multicast group when handover occurs. This means handover will in-troduce long shutdown of connection, because not only han-dover process in layer 2 but also multicast join and multi-cast tree reconstruction is necessary. This causes loss of large amount of packets and finally causes NAK implosion. In this paper, we claim that packet forwarding which is originally applied to unicast communication can be applied to multi-cast communications and resolve this NAK implosion prob-lem caused by handover. Our simulation results show that packet forwarding mechanism significantly improves scala-bility of NAK suppression-base reliable multicast protocol. Keywords: wireless multicast, reliable multicast, NAK sup-pression, handover, packet forwarding
1
Introduction
Multicast communication can support dissemination of the same data to potentially large number of receivers. IP multi-cast[1]is the multicast protocol which supports network-layer multicasting in the Internet. In IP multicast, a source sends a single data packet whose destination IP address is the cor-responding multicast group. A data packet is replicated at an adequate router and forwarded to an interface below which there are receivers. IP multicast uses UDP in the transport layer, so originally it does not support any reliability. To support reliability, reliable multicast protocol[2][3] should be implemented in the transport layer. In reliable multicast, re-transmission of lost packet is necessary for maintaining reli-ability. In multicast communication, a lot of participants are included in general. This character of multicast communica-tion easily leads to overload of retransmission process at the sender, receivers and network nodes. For example, concentra-tion of control packets, acknowledgement packets(ACKs) or negative acknowledgement packets(NAKs) sent to the sender degrades performance of the sender. This problem is well
known as the “feedback implosion problem” (ACK implo-sion or NAK imploimplo-sion). A number of feedback suppresimplo-sion mechanisms have been proposed to deal with this problem. One of the most popular and effective solutions is NAK sup-pression mechanism used in SRM(Scalable Reliable Multi-cast)[4]. NAK suppression mechanism uses timer-based ap-proach. Whenever a receiver detects a packet loss, it sched-ules a pending NAK transmission at a randomly chosen point of time in the future. If the receiver receives a NAK generated by another receiver, it cancels its NAK transmission. This mechanism is called NAK suppression. When a timer expires without receiving a NAK from other receivers, the receiver multicast a NAK. NAK suppression mechanism reduces the number of generated NAKs[6], which brings good scalability to reliable multicast protocol. So, we focus on reliable multi-cast protocol with NAK suppression mechanism.
Today, it is natural to use wireless access for accessing the Internet and mobile communication environment is now be-coming general. This technical trend makes wireless multi-cast attractive. When reliable multimulti-cast is served in wireless environment, several technical problems arise. One of the most serious problems is packet loss in wireless networks. Causes of packet loss in wireless network can be categorized into wireless-link-caused loss and handover-caused loss[7]. Wireless-link-caused loss is packet loss caused by low quality of wireless link. Handover-caused loss is packet loss caused by handover operation. The latter packet losses have more significant impact to reliable multicast in wireless commu-nications, because handover causes long-period bursty loss. This bursty loss may cause tentative serious increase of gener-ated NAKs and leads to performance degradation due to NAK implosion. This paper focuses on this handover-caused loss in wireless reliable multicast.
In unicast communication, packet forwarding[8] can re-solve handover-caused packet loss. When handover of mobile host is predicted, an access point caches some packets during handover period, and forwards this cached packet to a new access point of the corresponding mobile host. In wireless multicast, however, this packet forwarding is not easy to be applied. This is because mobile host will leave an old access point by leaving the multicast group and join to a new access point by re-joining to the corresponding multicast group. A new access point only manages multicast group, i.e. existence of at least one host which has an interest of receiving the cor-responding multicast group datagram beneath it, and does not manage all hosts of a multicast group. This feature makes it a little difficult to apply packet forwarding mechanism to
wire-less reliable multicast. In this paper, we claim that packet for-warding mechanism brings significant performance improve-ment to wireless reliable multicast even though it has some disadvantage that an access point should manage all nodes beneath it. We think this kind of host management will re-solve deployment problems of IP multicast[9] and be benefi-cial from the viewpoint of security and accounting reasons as well. Our simulation results show that packet forwarding in wireless reliable multicast improves scalability significantly, compared with reliable multicast protocol without packet for-warding.
Remainder of the paper is structured as follows. Section 2 shows overview of reliable multicast and its technical prob-lems. Section 3 explains mobile multicast and influence of mobility to reliable multicast. This section also includes brief introduction of packet forwarding. Section 4 presents packet forwarding mechanism in wireless reliable multicast. In sec-tion5, we evaluate packet forwarding in wireless reliable mul-ticast and show its improvement in scalability. Finally, section 6 concludes the paper.
2
Reliable Multicast Protocol
IP multicast provides an efficient way to disseminate the same data to potentially large number of receivers. How-ever, IP multicast generally uses UDP in the transport layer, which means it does not provide any support for reliable data dissemination. To support reliability, retransmission control, congestion control and flow control should be implemented at the transport layer. A number of proposals for reliable mul-ticast protocols have been proposed so far[2]. In this section, we briefly explain about reliable multicast protocols and show our simple protocol model which we focus on in the paper.
2.1
NAK-based or ACK-based
There are two approaches for retransmission control, an ACK-based one and a NAK-based one. For an ACK-based approach, all receivers send an ACK to the source, so ACK implosion is a serious problem. For a NAK-based approach, only receivers suffering packet loss send a NAK to the source, so the number of feedback packets should be small than an ACK-based approach. Both approaches are mathematically analyzed and evaluated from the viewpoint throughput per-formance in [10] and from the viewpoint of delay perfor-mance in [6]. Perforperfor-mance evaluation results in these papers show that a NAK-based approach has better scalability than an ACK-based one. Simulation results in [5] show that an ACK-based approach has worse scalability even though a hi-erarchical structure, e.g. RMTP(Reliable Multicast Transport Protocol)[11], is applied.
Even though a NAK-based protocol has better scalability than an ACK-based one, NAK implosion may cause perfor-mance degradation when multicast group size is large. To resolve this implosion problem, several approaches have been proposed. SRM(Scalable Reliable Multicast)[4] applies a timer-based approach. Whenever a receiver detects a packet loss, it
schedules a pending NAK transmission at a randomly chosen point of time in the future. If the receiver receives a NAK generated by another receiver, it cancels its NAK transmis-sion. This mechanism is called NAK supprestransmis-sion. When a timer expires without receiving a NAK from other receivers, the receiver multicast a NAK. With this NAK suppression mechanism, the number of generated NAKs is significantly reduced[6], which brings good scalability. Because of its good scalability, we focus on NAK-based approach, espe-cially a NAK suppression protocol, in this paper.
2.2
NAK Suppression Protocol
In this paper, we focus on wireless handover issues in wire-less reliable multicast communications. It is not our purpose of this paper to develop a specific protocol, so we use a gen-eral protocol model for NAK-based protocol and applies it to mobile communication environment. General NAK-suppression protocol behaves as follows.
• The sender multicast all packets to all receivers. • Whenever a receiver detects a lost packet, it schedules a
pending NAK transmission at a randomly chosen point of time in the future. In the meantime, if the receiver receives a NAK(generated by another receiver) for this packet, it cancels the transmission of its own pending NAK(NAK suppression).
• If, at the scheduled time for a pending NAK
transmis-sion, a NAK for this packet has not been received since the pending NAK transmission was first scheduled, the receiver multicast a NAK to the sender an all other re-ceivers, and starts a timer.
• When a receiver receives a NAK for a packet that it has
not yet received and it has initiated the random delay prior to sending a NAK, the receiver sets a timer and behaves as if it had sent the NAK.
• The expiration of a timer without prior reception of the
corresponding packet causes the receiver infers that a packet has been lost, and operates as it detects a packet loss (as in item2 above).
• Whenever the sender receives a NAK, it re-multicast a
corresponding packet to all receivers.
3
Wireless Multicast
In this section, we discuss about multicast communications in wireless networks and also describe about reliable multi-cast in wireless networks.
3.1
Multicast Communications in Wireless
Networks
In today’s Internet, a multicast member can access to the Internet via not only wired access but also wireless access. Nomadic users require a new kind of network host that can
Figure 1: Wireless Mulitcast
move from one access point to another easily. We call this kind of host as a mobile host(MH). For mobility management in the Internet, Mobile IP[12] has been proposed. When a MH moves to a new subnet(foreign network), care-of-address(CoA) is assigned temporally to this MN. The network portion of CoA matches that of the foreign network, which means by CoA a datagram can be forwarded to a foreign network by ordinary IP routing. A MN also has its IP address in its per-manent home network, home address. A CoA of MN is reg-istered at the agent located at the home network, which is called home agent. When a correspondent node would like to communicate with a MN, it can forward a datagram to its home address. When this datagram reaches a home network, a home agent can identify the CoA of MN and forwards this datagram to a foreign network by encapsulation technique.
For mobility management in wireless multicast, two tech-niques have been proposed, bidirectional tunneling and re-mote subscription[13](Fig.1). Bidirectional tunneling looks like Mobile IP for unicast traffic. In bidirectional tunneling, a home agent receives multicast datagram and forwards it to CoA of MN by making use of unicast tunneling. By this method, mobility of MN can be hidden to the sender(correspondent). However, it uses unicast tunneling from the home network to a MN, so redundant path caused indirect routing wastes net-work resources.
Remote subscription is a simple approach for mobility man-agement in wireless multicast. When a MH moves to a new subnet, it obtains its CoA of this foreign network. The MH re-joins a multicast group by using this CoA. For re-join, modi-fication of IGMP for Mobile IP, MLD(Multicast Listener Dis-covery)[14] is used. In remote subscription, delay for re-join may be a little large because it needs not only general Mo-bile IP process but also multicast tree construction process. However, it can achieve efficient usage of network resource because it does not use unicast tunneling which is necessary for bidirectional tunneling. So, remote subscription should be preferable approach for wireless multicast which potentially includes many mobile multicast hosts.
3.2
Reliable Multicast in Wireless Networks
When remote subscription is applied for wireless multi-cast communications, a mobile multimulti-cast receiver suffers from
Figure 2: Scalable NAK Suppression Protocol with Packet Forwarding
handover delay when it moves towards a new subnet. This is because not only handover operation, i.e. obtaining CoA, but also multicast joining operation such as join latency and tree reconstruction delay is necessary.
In remote subscription, this delay highly depends on whether there exists another receiver(s) of the corresponding multicast group in a new subnet. When there exists another receiver in a new subnet, multicast tree has already a branch to this subnet and multicast datagram has already come there. In this case, handover delay is not so large. When there exists no receiver in a new subnet, multicast joining operation takes a little time period, which leads to bursty packet loss. In this paper, we call this kind of busrty packet loss a handover loss.
When this handover loss occurs, NAK suppression mech-anism may not work well. This is because during handover period a mobile node cannot receive not only a data packet but also NAK sent by other receivers. In this case, a NAK is multicast to all other members of the corresponding multicast group from the mobile node.
Even in the case that another multicast member exists in a new subnet, when a mobile host moves from a subnet where wireless channel quality is bad and to a new subnet where wireless channel quality is good, packet losses detected at a mobile host may be packet losses occurred at the old subnet. When the source multicast these lost packets to all members, receivers in this new subnet which correctly receive these pack-ets should receiver them, which leads to inefficient network resource usage.
From these reasons, handover loss should not be recovered globally and should be recovered locally. From this view-point, we propose a new packet recovery mechanism for wire-less multicast, packet forwarding, which will be explained in the next section.
4
Wireless Reliable Multicast with Packet
Forwarding
As an effective way to resolve handover loss discussed in the previous section, we claim that packet forwarding tech-nique which is originally proposed for wireless TCP[8] can be a good solution(Fig.2). General NAK suppression proto-col applied packet forwarding behaves as follows.
• The sender multicast all data packets to all receivers in
a multicast group.
• A receiver detects a packet loss when there is a gap of
sequence number of received packets and recovers this packet loss using NAK suppression protocol described in Section 2.2.
• When a mobile host detects packet loss during
han-dover operation(exactly this can be detected just af-ter the handover operation), it uses packet forwarding mechanism described below.
– When an access point predicts(or detects) han-dover of a mobile multicast host, it caches arrived packets of the corresponding multicast group. – After handover operation completed, a mobile
mul-ticast host sends the transmission demand packet to the old access point.
– When the old access point receives this request, it forwards cached packets to the mobile multicast host.
• When the access point receives a demand packet and it
does not have cached data of the corresponding multi-cast group, it sends a control packet notifying this.
• When a mobile multicast host receives this control packet,
it starts ordinary NAK suppression protocol.
In order to apply packet forwarding technique towards wire-less multicast, an access point should identify which mobile host(mobile multicast host) exists beneath it. This is because it should identify mobility(handover) of each mobile multi-cast host in order to start caching mechanism. In general, an access point has only a necessity to manage existence of a least one multicast member. Thus, for packet forwarding technique, overhead for an access point increases, but we be-lieve this overhead is acceptable because of the following two reasons. One reason is that as shown in the next section per-formance improvement obtained by packet forwarding tech-nique is significant. The other reason is that for security and accounting reason management of mobile hosts will be man-aged by the network. For security and accounting, all mem-bers of each multicast group should be managed. When this kind of management is implemented, our proposed scheme can be easily deployed because management of all mobile hosts for each multicast group can be realized.
Reliable multicast protocol is implemented in transport layer of end hosts and our proposed packet forwarding method is implemented in network layer of an access point. It is de-sirable that packet forwarding method is only applied to reli-able multicast traffic. In order realize this operation, transport layer processing is necessary at an access point. Active net-work technology[17] which is a new netnet-work architecture and realizes higher layer operation than network layer, is a candi-date technology which enables this transport layer operation at an access point.
Figure 3: Parameters in Simulation
Figure 4: Network Topology for Simulation (Tires Model)
5
Performance Evaluation
We evaluate performance of packet forwarding technique applied for NAK suppression protocol by computer simula-tion. For evaluation, we especially focus on scalability.
5.1
Simulation Model
For network topology model, we use a sophisticated model, Tiers model[15](Fig.4). For LAN, we assume two kinds of networks, one is wired and the other is wireless. 50% of LANs in generated topology are assumed to be wireless LAN. Location of wireless LAN is located randomly in the network. Multicast hosts are connected to MAN router with probabil-ity of 0.2 and LAN(wireless and wired) with 0.8. We assume one static host as the source of a multicast group. Loss prob-abilities and other simulation parameters are shown in Fig.3. Packet loss probability is assigned to each Tier from mea-sured results reported in [16]. For mobility of mobile mul-ticast hosts, distribution of their mobility speed is truncated exponential distribution in 4 − 100[km/h] with average of
10[km/h].
For handover delay, 1[sec] handover delay is assumed for the case that another multicast member does not exist, and
0[sec] is assumed in the case that another member exists
al-ready in a new network. This means Layer 2 handover delay is assumed to be 0. And Layer 3 handover delay is assumed to be 0 in the situation that another member exists already. It is easily predicted that our proposed packet forwarding method has better performance for higher handover delay. Thus, sim-ulation results obtained under the condition that L2 handover is 0, will give the lower bound of the performance of our pro-posed scheme. When handover of a mobile multicast host is
predicted(detected), an access point caches arrived packet of the corresponding multicast group during 2[sec].
Reliable multicast protocol is implemented in transport layer of end hosts and our proposed packet forwarding method is implemented in network layer of an access point. It is de-sirable that packet forwarding method is only applied to reli-able multicast traffic. In order realize this operation, transport layer processing is necessary at an access point. Active net-work technology[*****] which is a new netnet-work architecture and realizes higher layer operation than network layer, is a candidate technology which enables this transport layer oper-ation at an access point.
5.2
Simulation Results
Figure 5 shows average delay characteristics of NAK sup-pression protocol without Packet Forwarding mechanism and NAK suppression protocol with Packet Forwarding mecha-nism. Delay is normalized with one-way delay for ideal trans-mission, i.e. transmission delay of ideal case transmission with no queueing delay at a router. Horizontal axis shows the number of receivers in a multicast group. As shown in this figure, normalized delay is slightly improved with Packet For-warding mechanism. This is because with Packet ForFor-warding lost packets are forwarded from not the source but the old ac-cess point which is generally located closer than the source.
As shown in Fig.5, average delay characteristics of general NAK suppression protocol saturates when the number of re-ceivers is around 400. With Packet Forwarding mechanism, delay performance saturates where the number of receivers is around 700. These saturations of delay characteristics are caused by NAK implosion at the sender. This means from the scalability viewpoint Packet Forwarding improves scalability of NAK suppression protocol significantly because NAK sup-pression protocol with Packet Forwarding mechanism can in-clude more receivers.
Figure 6 shows the total number of hops for data pack-ets, NAKs and transmission demand packets(only for Packet Forwarding). This total number of hops can be used as per-formance metric for efficient usage of network resource. As shown in Fig.6, Packet Forwarding mechanism decreases this total number of hops. In NAK suppression protocol with-out Packet Forwarding mechanism, lots of NAKs are multi-cast to all members when handover loss occurs. However, with Packet Forwarding mechanism, a transmission demand packet is unicast to the old access point and lost packet which are cached at the old access point can be forwarded to the mobile multicast host by unicast. On the contrary, in NAK suppression mechanism, not only NAKs but also retransmit-ted packets are also multicast to all members, which leads inefficient usage of network resources.
Figures 7 and 8 show similar characteristics of Figs. 5 and 6, but they show characteristics for only lost packets dur-ing handover period. For example, Fig.7 shows average de-lay characteristics measured only for lost packets during han-dover period. As shown in these figures, Packet Forward-ing mechanism brForward-ings significant performance improvement
to handover packet loss from the viewpoint of both delay per-formance and network resource usage. Figure 9 shows sim-ilar characteristics of Fig.8, but it shows characteristics for packets excluding lost packets(this means packets not aver-aged in Fig.8). As shown in Fig.9, even for not lost pack-ets some performance improvement is obtained. This is be-cause redundant packets generated for retransmission is re-duced by our proposed packet forwarding, which leads to performance improvement for not lost packets. These re-sults show that our proposed packet forwarding brings per-formance improvement not only for lost packets but also cor-rectly received packets.
In our performance evaluation thus far, we use homoge-neous model for wireless networks, i.e. packet loss in wireless network occurs similarly in every network. Here, we evaluate our proposed packet forwarding mechanism in the situation that there are several WLAN in bad quality. We assign packet loss probability of 0% and 10% by turns. In this case, a mo-bile host may move towards good quality WLAN form bad quality WLAN. Figure 10 shows normalized delay charac-teristics of our proposed packet forwarding and general NAK protocol in this situation(“heterogeneous” in the figure). Sim-ulation results for homogeneous model is also shown in the figure(This part is the same as Fig.5). As shown in this figure, general NAK protocol saturates with smaller number of re-ceivers compared with performance results in homogeneous model. For our proposed packet forwarding mechanism, per-formance results almost the same as the homogeneous model. When a mobile host moves to WLAN with good quality, it re-ceives further packets than its last received packet. However, our packet forwarding mechanism will forward these packets and prevent redundant generation of NAKs.
6
Conclusions
In the paper, we discuss how bad handover packet loss is for wireless reliable multicast. We claim that packet forwarding which recovers packet loss caused by handover by forward-ing cached packets from the old access point to the mobile multicast host, will bring significant performance improve-ment. This is because packet forwarding mechanism only needs unicast transmission of forward request packet and uni-cast transmission for lost packets(cached packets). On the contrary, general NAK protocol requires multicast transmis-sion of NAKs and multicast retransmistransmis-sion from the source. Our simulation results show that Packet Forwarding mech-anism brings significant performance improvement from the viewpoint of scalability and efficient network resource usage.
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Figure 5: Normalized Delay Charac-teristics NAK protocol with PF Number of Receivers To tal n umbe r o f h op 0 0.5 1 1.5 200 400 600 800 (×107 )
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